The present invention relates to a fuel cell separator and a polymer electrolyte fuel cell, and particularly to a high quality fuel cell separator capable of reducing elution of ions and organic matters and exhibiting a high moldability and a high dimensional stability, and a high performance polymer electrolyte fuel cell using the fuel cell separators, which is capable of ensuring a stable output without reduction in output during operation of the fuel cell, thereby improving the operational efficiency thereof.
A fuel cell, particularly, a polymer electrolyte fuel cell is configured as a cell stack composed of an array of unit cells of the number of several tens to several hundreds, wherein each of the unit cells includes, as shown in FIG. 1, a pair of fuel cell separators 1 each having on its right and left side surfaces a plurality of ribs 1a, and a polymer electrolyte membrane 2 and two gas diffusion electrodes 3 interposed between these separators 1.
As shown in FIG. 2, the fuel cell separator 1 has a unique shape having the plurality of ribs 1a projecting from the right and left side surfaces of a thin plate-like body, wherein passages (grooves) 4 for supplying and discharging a fuel gas such as hydrogen or oxygen are formed between the ribs 1a on each side of the separator 1 and the corresponding electrode 3. Accordingly, the separator 1 is required to have a high elasticity, an excellent dimensional accuracy, and a desirable gas non-permeability, and also the unit cell (fuel cell) is required to have a high gas sealing characteristic capable of preventing a leak of a fuel gas, and an excellent impact resistance, particularly, when the fuel cell is used as a movable power source for automobiles, etc.
Such a fuel cell separator has been produced, for example, by a method (1) of kneading a carbon powder as a raw material with a phenol resin as a binder, molding and sintering the mixture, and carbonizing and graphitizing the sintered mixture (disclosed, for example, in Japanese Patent Laid-open No. Hei 8-222241), and a method (2) of molding a composition containing graphite and a binder such as a phenol resin.
By the way, since a voltage outputted from each unit cell of a fuel cell is low, the fuel cell must be composed of an array of unit cells of the number of several tens to several hundreds for ensuring a practical output ( less than several hundreds kW). Accordingly, to uniformly align the unit cells, fuel cell separators used therefor, each having a uniform shape with no deformation and no partial unevenness in thickness, have been required to be produced with a high dimensional accuracy and a high moldability.
The above-described method (1), however, has a problem that since a graphite block is produced by carbonization and graphitization and is then formed into a separator having a desired shape by machining, the production steps are complicated and the production cost is raised, and also it is difficult to obtain a uniform separator having a high dimensional accuracy.
The above-described method (2) has also a problem that since the fuel cell separator is produced by using novolak type phenol as a phenol resin and hexamine as a hardening agent, ions and organic matters such as free phenol, formalin, ammonia, hexamine as the hardening agent are eluted from the fuel cell separator due to water produced during operation of the fuel cell, so that the electric conductivity of the produced water is raised, to lower the output the fuel cell, thereby degrading the operational efficiency of the fuel cell.
An object of the present invention is to provide a high quality fuel cell separator capable of reducing elution of ions and organic matters and exhibiting a high moldability and a high dimensional stability, and a polymer electrolyte fuel cell using the above fuel cell separators as part or all of separators in the fuel cell, which cell is capable of ensuring a stable output after a long-term operation and a high operational efficiency and which is particularly suitable as a movable power source for automobiles, small-sized marine vessels, etc.
To achieve the above object, the present inventor has made studies of improvement of a fuel cell separator molded from a fuel cell separator composition mainly containing a conductive material and a binder, and has found that the elution of ions and organic matters from the separator due to water produced during operation of a fuel cell using the separator can be significantly reduced by using, as the binder, a phenol resin composed of a mixture of novolak type phenol and resol type phenol.
The reason why the use of the mixture of novolak type phenol and resol type phenol as the binder of the fuel cell separator composition mainly containing a conductive material and a binder is effective to prevent the elution of ions and organic matters from the separator due to water produced during operation of the fuel cell, is that the resol phenol acts as a hardening agent for the novolak type phenol, and since the novolak type phenol is hardened by condensation reaction, ammonia does not occur during the reaction unlike the case of using hexamine as a hardening agent for the novolak type phenol, and that the elution of ions and organic matters from the resol type phenol is very small as compared with the novolak type phenol.
The present inventor has eventually found that the fuel cell separator molded from a composition using, as a binder, the mixture of resol type phenol and novolak type phenol is effective to suppress the elution of ions and organic matters such as free phenol, formalin, ammonia and resol type phenol as a hardening agent as much as possible, and more specifically, reduce the elution degree, expressed by the electric conductivity, to 50 xcexcS/cm or less, and also to enhance the dimensional accuracy and moldability and ensure a uniform quality. It is to be noted that the elution degree, expressed by the electric conductivity, is evaluated by putting 3.5 g of a test piece cut from the separator in 305 mL of pure water, heating the water at 90xc2x0 C. for 500 hr, and measuring the electric conductivity of the water. On the basis of the above knowledge, the present invention has been accomplished.
Further, the present inventor has found that a polymer electrolyte fuel cell using the above-described fuel cell separators, each of which is capable of reducing the elution of ions and organic matters and exhibiting a high moldability and a high dimensional stability, as part or all of separators in the fuel cell is effective to suppress the reduction in output after long-term operation and exhibit a high operational efficiency, and is particularly suitable as a movable power source for automobiles, small-sized marine vessels, etc.
According to a first aspect of the present invention, there is provided a fuel cell separator molded from a fuel cell separator composition mainly containing a conductive material and a binder, characterized in the separator is specified such that after 3.5 g of a test piece cut from the fuel cell separator is put in 305 mL of pure water and the water is heated at 90xc2x0 C. for 500 hr, an electric conductivity of the water is 50 xcexcS/cm or less.
According to a second aspect of the present invention, there is provided a fuel cell separator molded from a fuel cell separator composition mainly containing a conductive material and a binder, characterized in that the binder comprises a phenol resin obtained by adding 20 to 100 wt % of resol type phenol to novolak type phenol.
In the above separator, preferably, 50 parts by mass or less of the binder is added to 100 parts by mass of the conductive material, and more preferably, the separator is specified such that after 3.5 g of a test piece cut from the fuel cell separator is put in 305 mL of pure water and the water is heated at 90xc2x0 C. for 500 hr, an electric conductivity of the water is 50 xcexcS/cm or less.
According to a third aspect of the present invention, there is provided a polymer electrolyte fuel cell composed of an array of a plurality of unit cells, each of which includes a pair of electrodes with a polymer electrolyte membrane put therebetween and a pair of separators having gas supplying/discharging passages with the electrodes put therebetween, characterized in that each of part or all of the separators of the fuel cell is molded from a fuel cell separator composition mainly containing a conductive material and a binder; and the separator is specified such that after 3.5 g of a test piece cut from the fuel cell separator is put in 305 mL of pure water and the water is heated at 90xc2x0 C. for 500 hr, an electric conductivity of the water is 50 xcexcS/cm or less.
According to a fourth aspect of the present invention, there is provided a polymer electrolyte fuel cell composed of an array of a plurality of unit cells, each of which includes a pair of electrodes with a polymer electrolyte membrane put therebetween and a pair of separators having gas supplying/discharging passages with the electrodes put therebetween, characterized in that each of part or all of the separators of the fuel cell is molded from a fuel cell separator composition mainly containing a conductive material and a binder; and the binder of the separator comprises a phenol resin obtained by adding 20 to 100 wt % of resol type phenol to novolak type phenol.
In the above polymer electrolyte fuel cell, preferably, 50 parts by mass or less of the binder is added to 100 parts by mass of the conductive material, and more preferably, the separator is specified such that after 3.5 g of a test piece cut from the fuel cell separator is put in 305 mL of pure water and the water is heated at 90xc2x0 C. for 500 hr, an electric conductivity of the water is 50 xcexcS/cm or less.